The present invention relates to optical detection for low optical density web and, more particularly, but not exclusively to such optical detection that is suitable for all optical densities including the very low densities, without requiring adjustments. The optical detection system is preferably suitable for inclusion within a web feeder system and in particular to inclusion within the buffer of a variable speed web feeder system.
In web feeder systems it is necessary to sense the presence of the web so that the feeder system knows whether the web is being fed correctly. Sensing of web at the wrong location, or failure to sense the web at the correct location can be used as a trigger to interrupt the feed.
Furthermore the web feeder system typically feeds the web to a web utilizing component located downstream of the feeder system. The web utilizing component is typically but not exclusively a printing engine. The web utilizing component may require temporary variations in the rate at which the web is fed. A printer may require the web to be stopped at certain locations before starting a new printing image. In certain cases the web may actually be moved backwards.
Alternatively certain parts of a complex layout may require different feed rates. Thus a layout including a banner headline followed by a high resolution image would require the web to be fed relatively fast while the banner part is printed and relatively slowly while the high resolution image is printed.
Generally the web feeding rollers at the printer react rapidly to any change in feed requirements. However the web feeder system reacts more slowly to these changes and thus web buffering capacity is provided to match between the two. Typically the web buffering capacity is provided by a component known as a vacuum box. The vacuum box sits in the web feeding path and takes up additional web in the form of slack. A loop of the web enters the vacuum box at a first end and is pulled by a vacuum towards a second end in such a way that the length of the loop towards the second end can be varied according to the instantaneous need for buffering. The variability in the length of the loop serves to buffer the web, so that instantaneous changes in feed rate can be absorbed by adding to or reducing from the loop.
In order for the vacuum box to work, it is necessary for the current length of the loop to be known to the feeding control apparatus, so that the length can be controllably varied. Contact sensors are not ideal since the loop does not generally come into contact with the walls of the box but rather sits in the space within the box where it is drawn to the far wall by vacuum. Non-contact sensors are typically used and these include both optical sensors and ultrasound sensors. Optical sensors work by shining light from a sensor at one side of the box through the space of the box to a reflector at the far end. If the loop is absent then a return beam is sensed. If the loop is present then no return beam is sensed since the light never reaches the reflector.
A difficult with optical detection is that the web being fed is not necessarily opaque but can be of any optical density. Web materials include many transparent materials, and in some cases feeders may be required to feed different materials at different times, the materials having different levels of opacity.
One solution to the above is to increase the sensitivity of the optical detectors so that even the relatively minimal light scattering brought about by the most transparent web will be detected. A difficulty with this approach is that it is very susceptible to noise. The difference between a light level indicating detection and that indicating non-detection is small and therefore noise due say to light dust may lead to false detections. Furthermore, for a feeder that is used for feeding web of different opacity, the user would be advised to alter the sensitivity depending on the web material being fed. Such a requirement goes against the overall aim of making the system simpler to use.
Another solution is to dispense with optical detection altogether and use ultrasound. Ultrasound has the advantage that all web materials are sufficiently opaque to the signal but the detectors are expensive, and three detectors are generally required per vacuum box in order to make the control system effective.
There is thus a widely recognized need for, and it would be highly advantageous to have, a detector system for a web buffer which is devoid of the above limitations.